Some work remains to be done. At this point OpenJDK 6 doesn’t quite
pass all of the compatibility tests in the Java SE 6 JCK, but it’s
very close. It also doesn’t yet have a MIDI synthesizer, or support for SNMP.
Despite these gaps OpenJDK 6 is, nonetheless, highly usable—it can run
NetBeans and Eclipse, and GlassFish and JBoss.

One more thing
To celebrate this milestone we’ve redesigned the OpenJDK front page. The new layout includes
a main panel which refers people who are looking for prepackaged binaries,
documentation, or tutorials to the right place. Below that there’s a rolling
summary of recent announcements and the latest batch of entries from Planet JDK. Finally, there’s a new installation page which gives
step-by-step instructions for installing OpenJDK-based packages in the
distributions that provide them; we’ll expand this as more distros ship such
packages.

Mark, this is excellent news for the software world! I've been very impressed with what I've seen so far of OpenJDK 6.

I've also been keeping an eye on the Java SE 6 Update 10 progress (I have the new plugin enabled on this browser right now.) I'm looking forward to seeing u10 show up in the OpenJDK.

If the new plugin was added to the OpenJDK it would fill a big hole that you hadn't mentioned above. (Although the icedtea-gcjwebplugin is very capable it doesn't have the full compatibility that some sites need.)